1. Field of the Invention
This invention relates to devices containing epitaxial oxide thin films, and in particular, devices having electrically conductive oxides.
2. Art Background
Many electrical and optical devices rely on deposited thin oxide films. For example, thin films of superconducting oxides such as YBa.sub.2 Cu.sub.3 O.sub.7 have been proposed for electronic devices such as SNS (superconductor-normal metal-superconductor) Josephson junctions and regions of normal metallic conductivity in an integrated superconducting device. Similarly, ferroelectric oxide materials such as Pb(Zr.sub.0.52 Ti.sub.0.48)O.sub.3 (PZT) have been proposed for use in ferroelectric memory devices.
In forming thin film devices, a series of layers are deposited on a single crystal substrate. Typically, a thin film device has, in addition to the active region, (e.g. an optically, electrically, or magnetically active area) regions having dielectric properties and regions having relatively high electrical conductivity. (Relatively high electrical conductivity in the context of this invention is a resistivity less than 1000 .mu..OMEGA.-cm.) The conductive regions are generally employed to form contact to and interconnections between active regions while the dielectric regions are required to provide electrical isolation between different conductive and/or active regions. As shown in FIG. 1 to produce electrical conductivity between active regions 12 and 14, it is necessary to have equally high conductivity throughout the conductive material forming interconnect region 15 and contact regions 10. Thus, for many electrically conductive regions such as contact regions, it is quite desirable or even necessary that the conductivity be isotropic, i.e., the conductivity along the z-axis (an axis perpendicular to the major surface of the thin film), as well as two other mutually perpendicular axes have a mean deviation of less than 300% over the operating temperature range.
The characteristics of the device, however, rely on more than just the chemical composition of the constituent layers. To obtain the desired properties such as electrical properties, these layers, (e.g., the oxide materials, such as the ferroelectric oxide material or the superconducting oxide material) must be epitaxially deposited. (Epitaxial growth in the context of this invention is defined as there being no more than three discrete crystallographic orientations of one layer with respect to the adjoining material upon which it is deposited.) Generally grain boundaries, crystal defects, and interface defects between the layers resulting from non-epitaxial growth are undesirable. For example, non-epitaxial growth leads to high angle grain boundaries in materials such as PbZr.sub.0.52 Ti.sub.0.48 O.sub.3, which in turn, result in device degradation through aging and fatigue induced by charge segregation and decay at the grain boundaries. Similarly, lack of epitaxial growth in the deposition of copper oxide based superconductor materials leads to significantly decreased critical current densities and degraded device properties. Interfaces between conductive layers and active layers should also be free of objectionable defects and exhibit coherent crystal structure. The presence of contamination or incoherency at an interface with, for example, copper oxide based superconductors leads to high interfacial resistance and an additional insulating interfacial region. Interfacial problems in ferroelectric devices often lead to degraded performance characteristics.
Thus, it is quite desirable to have an isotropically conductive oxide material that 1) grows epitaxially on substrates and/or other oxides, 2) allows epitaxial growth of other oxides deposited upon it, and 3) does not adversely undergo chemical reactions with these other oxides. Conductive oxide materials proposed for device applications include Sr.sub.2 RuO.sub.4 Lichtenberg, F., et al., Applied Physics Letters, 60, 1138 (1992), PrBa.sub.2 Cu.sub.3 O.sub.7, and isotropic perovskites such as Nb-doped SrTiO.sub.3. However, the first has never been grown as a film, and is a layered, highly anistropic material; the second is semiconductive and anistropic; and the third is also extremely difficult to grow reproducibly while being incompatible with oxidizing conditions used to form oxides such as YBa.sub.2 Cu.sub.3 O.sub.7. Therefore, an improved oxide having isotropic electrical characteristics is still not available.